Background and Objectives: This in-vitro study was conducted to evaluate the strength and properties of Type III and Type IV gypsum mixed with disinfectant solutions.Materials and Methods: Type III and Type IV gypsum were used for the study. Three different mixing solutions namely waterqueous solutions of 0.525% sodium hypochlorite and 2% glutaraldehyde were used. Gypsum materials were subjected to further modification by adding a mixture of 1.0% gum arabic and 0.132% calcium hydroxide before mixing with the disinfectant solutions, at two different liquid/powder (L/P) ratios for each. Both, the unmodified and the modified gypsum were tested for compressive and tensile strength after one hour and one week from the start of the mix. The crystalline configuration of the fracture fragments of the unmodified and modified set gypsum were studied under the scanning electron microscope.Results: The disinfectant solutions reduced the strength of both Type III and Type IV gypsum. Water showed higher-strength, which was followed by 0.525% sodium hypochlorite and 2% glutaraldehyde. The modified Type III and Type IV gypsum with reduced L/P ratio also showed strength values less than that of the control groups. Interpretation and Conclusion: Chemical disinfectants reduced the strength of gypsum when used as water substitutes. Gum Arabic and calcium hydroxide additives permitted lower L/P ratio, however, there was still excess water retained in the set gypsum that lowered the strength values of Type III and Type IV gypsum. Hence, further reduction of L/P ratio may increase the properties of the modified Type III and Type IV gypsum.

How to cite this article:Roy SM, Sridevi J, Kalavathy N. An evaluation of the mechanical properties of Type III and Type IV gypsum mixed with two disinfectant solutions. Indian J Dent Res 2010;21:374-9

Dental impressions are potential sources of microorganisms as they are contaminated with patient's saliva and blood that can cross-infect stone casts poured. [1]

Gypsum casts made from contaminated impressions can also be a medium for cross-contamination. [2] The spray technique for disinfecting the surface of stone casts appears to eliminate the surface detail and reduces the strength. [3] The potentially damaging effects of the immersion technique, the difficulty in covering the entire surface of the cast with the spray disinfecting solution and the inability to assume that every impression has been disinfected have led to the need for incorporating a disinfectant directly into the gypsum. [4]

A study has reported that there will be decrease in the compressive strength with the addition of sodium hypochlorite to the stone. [5] The assumption is that the sodium ions from the hypochlorite interfere with the structure and strength of the gypsum, which contains calcium in its structure. [6]

Abdelaziz et al. [6],[7] , have suggested, modifying the gypsum with the addition of gum Arabic and calcium hydroxide before mixing with aqueous solutions of disinfectants, and the results have shown improvement of mechanical and surface properties of gypsum.

The present study was conducted to evaluate the effect of two commonly used disinfectants on the compressive and tensile strength of locally available Type III and Type IV dental gypsum, which was modified using 1% gum Arabic and 0.132% calcium hydroxide prior to mixing with the disinfectants.

Materials and Methods

Aqueous solutions of two disinfectants were diluted to the required concentration of 0.525% sodium hypochlorite and 2% glutaraldehyde. The Type III and Type IV dental stones were modified with the addition of 1% gum Arabic and 0.132% calcium hydroxide prior to mixing the powder and liquid.

The grouping of the samples were done as follows:

Group I: Type III unmodified stone with recommended L/P ratio

Group II: Type IV unmodified stone with recommended L/P ratio

Group III: Type III modified stone with recommended L/P ratio

Group IV: Type IV modified stone with recommended L/P ratio

Group V: Type III unmodified stone with reduced L/P ratio

Group VI: Type IV unmodified stone with reduced L/P ratio

Group VII: Type III modified stone with reduced L/P ratio

Group VIII: Type IV modified stone with reduced L/P ratio

Group I and II served as controls.

Each of these groups were further divided into three sub-groups according to the type of mixing solution used.

Subgroup 1: Dental stone mixed with water

Subgroup 2: Dental stone mixed with 0.525% of sodium hypochlorite

Subgroup 3: Dental stone mixed with 2% of glutaraldehyde

Thus, a total of 24 subgroups were considered for the study.

The Type III and Type IV dental stones were weighed and mixed with recommended amount of liquid using a vacuum mixer.

Samples were prepared according to American Dental Association specification number 25. [8]

The split metal molds that measured 5 mm in diameter and 10 mm in length [Figure 1] were greased with Vaseline and placed on a glass plate.

The mixed dental stone was poured into the split metal mold and placed on a vibrator. The mold was slightly overfilled and was covered with another glass plate held firmly on top and allowed to set for 30 min from the start of the mixing.

In this manner a total of 336 cylindrical specimens were prepared. Half of the specimens were tested for wet compressive strength and the other half were tested for dry compressive strength.

Wet strength

The specimens prepared for testing of wet strength were stored in a humidor at 23±20°C and 95±5% relative humidity for 2 h, and were then tested for wet strength. All the specimens for wet strength were tested on an Instron testing machine at a crosshead speed of 1 mm/min. The maximum load at which the cylindrical specimens were fractured was recorded. Fractures occurred completely in an oblique direction. The non-obliquely fractured specimens were discarded.

Dry strength

Specimens prepared for the dry strength testing were stored in a humidor at 23±20°C and 95±5% relative humidity for 24 h. Then the specimens were transferred to an incubator, which maintained a temperature of 37°C. The specimens were stored in an incubator for seven days and were tested for dry strength on an Instron testing machine

Testing tensile strength

The specimens for testing tensile strength were prepared using a split metal mold measuring 5 mm in diameter and 5 mm in length [Figure 1]. The prepared specimens were stored similarly as the specimens stored for the compressive strength testing. A total of 336 specimens were prepared and were divided into two halves of 168 specimens each and one half of the specimens were tested for wet tensile strength and other half for dry tensile strength using the same technique described for compressive strength.

Diametral tensile strength (DTS) was then calculated using the following equation: DTS=2F / πbd

where, F=breaking load

b=specimen height

d=cross-sectional diameter

Scanning electron microscopic study

The fractured fragments of the above tested specimens, which were approximately more than 5 mm and less than 10 mm in diameter were selected such that they can be held in hand comfortably during the examination.

An electrically conducting material, such as gold was sputter coated on to the fractured fragments. This was done to prevent the accumulation of static electric fields at the specimen due to the electron irradiation required during imaging. Another reason for coating is for obtaining high-resolution electronic imaging.

Specimens sputtered with gold were examined under scanning electron microscope at 3000Χ magnification. The SEM examination was done to study the crystalline configuration of the set-modified dental gypsum.

Results

The values obtained for different groups, after their strengths were tested using Instron testing machine, were statistically analyzed using ANOVA and multiple-comparison test.

Group I: Type III unmodified gypsum mixed with recommended L/P ratio

Water showed greater strength values when compared with sodium hypochlorite and glutaraldehyde. Sodium hypochlorite showed greater strength values when compared with glutaraldehyde with respect to dry compressive and tensile strength, however, it showed no significant difference with respect to wet compressive and tensile strength.

Group II: Type IV unmodified gypsum mixed with recommended L/P ratio

Water showed higher strength values when compared with sodium hypochlorite and glutaraldehyde. Sodium hypochlorite showed greater strength values when compared with glutaraldehyde except with respect to wet compressive strength where there was no significant difference.

Group III: Type III modified gypsum mixed with recommended L/P ratio

Water showed greater strength values when compared with sodium hypochlorite and glutaraldehyde with respect to dry tensile strengths, however, there was no significant difference with respect to the other values. Water showed greater strength values when compared with glutaraldehyde. Sodium hypochlorite showed greater strength values when compared with glutaraldehyde.

Group IV: Type IV modified gypsum mixed with recommended L/P ratio

Water showed greater strength values when compared with sodium hypochlorite and glutaraldehyde with respect to all strength values, except for dry compressive strength where there was no significant difference when compared with sodium hypochlorite. Sodium hypochlorite showed greater strength values when compared with glutaraldehyde with respect to all strength values, except with respect to wet tensile strength where there was no significant difference.

Water showed greater strength values when compared with sodium hypochlorite and glutaraldehyde with respect to all strength values, except for wet tensile strength where there was no significant difference when compared with sodium hypochlorite and glutaraldehyde. Sodium hypochlorite showed greater strength values when compared with glutaraldehyde with respect to all strength values, however, it showed no significant difference with respect to wet tensile strength.

Water showed greater strength values when compared with sodium hypochlorite and glutaraldehyde with respect to all strength values, except for dry tensile strength where there was no significant difference when compared with sodium hypochlorite.

Sodium hypochlorite showed greater strength values when compared with glutaraldehyde with respect to all strength values, however, it showed no significant difference with respect to wet tensile strength.

Scanning electron microscopic examination

The fractured specimens of all the test groups were collected to study the crystalline configuration of the set gypsum.

The fractured gypsum specimens were observed under SEM with a magnification of 3000Χ.

Crystalline configuration of various groups

Group I

Type III unmodified gypsum mixed with water in recommended L/P ratio: The crystals were prismatic, regular in shape and densely packed with little porosities.

Type III unmodified gypsum mixed with recommended sodium hypochlorite: The crystals were prismatic, regular in shape and with porosities.

Type III unmodified gypsum mixed with glutaraldehyde in recommended L/P ratio: The crystals were prismatic, regular in shape and with more porosity.

Group II

Type IV unmodified gypsum mixed with water in recommended L/P ratio: The crystals were cuboidal, regular in shape and densely packed with less porosity.

The SEM images of the unmodified and modified Type III and Type IV gypsum supported the results obtained by Instron testing machine.

Discussion

In prosthodontics, routine infection control protocols have been developed. Emphasis has been placed on the disinfection of impressions and casts used for the fabrication of prostheses. [2]

Leung and Schonfeld [2] observed the transfer of microorganisms from the impressions to the plaster casts that increased the risk of cross-contamination. Present guidelines recommend rinsing the impression under running water to remove saliva and immersion disinfection using any compatible disinfectant for varying lengths of time. [9] Most reports have found that dimensional stability is not significantly sacrificed with immersion techniques. However, problems may develop with the hydrocolloid and polyether materials because of imbibition of water.

The techniques recommended by the ADA for disinfecting dental casts include immersion in a suitable disinfectant or spraying with a suitable disinfectant solution. [9],[10] Some authors have also recommended the use of disinfectants as a water substitute for mixing gypsum. [6],[7],[11],[12]

Although these methods may provide a disinfected cast, the effect on the dimensional stability and physical properties of the set stone is questionable. [13]

If disinfectants are to be used in dental casts, a chemical that will maintain or improve the strength of set gypsum is needed. Trials to improve the strength properties of dental gypsum mixed with disinfectants have been reported. Based on these trials, additions of lignosulfonates, 1% potassium sulfate, calcium hydrogen phosphate, a liquid dispersing agent and microcrystalline additives have been recommended. [14],[15],[16] In addition, the studies of Abdelaziz et al., Sanad et al. and Alsadi et al., [6],[7],[14],[15] showed that a mixture of gum Arabic and calcium hydroxide reduces the amount of water necessary to mix gypsum products, resulting in improved strength properties.

The present study was done to explore the effect of selected disinfectants on the compressive and tensile strength properties of type III and type IV dental stones. This work is also concerned with the effect of gum Arabic and calcium hydroxide additives on these properties.

Results indicated that the substitution of disinfectant solution as mixing solution reduced the strength properties of the type III and type IV gypsum.

This reduction in the strength corroborates with the results of Ivanovski et al., and Abdaleziz can be explained as follows. Ions from sodium hypochlorite may alter the crystalline structure that could affect the crystals ability to intermesh. Glutaraldehyde disinfectant may be evaporated from the mixed materials, leading to incomplete solubility of some of the hemihydrate particles causing porosity within the set materials. [5]

Abdelaziz et al. Sanad et al. and Alsadi et al. [6],[7],[14],[15] found that the addition of gum Arabic and calcium hydroxide reduce the amount of liquid required and that after drying, it aided in adhesion between the hemihydrate particles, thereby enhancing the strength of the set gypsum.

However, in this study, the addition of 1% gum Arabic and 0.132% calcium hydroxide did not improve the strength of the type III and type IV gypsum, indeed, it had an adverse effect on both compressive and tensile strength.

The result suggested that even with the reduction of L/P ratio, the strength values were found to be less than that of the control. This result is in agreement with the study done by Abdelaziz, Attia and Combe [17] in which the reduction of L/P ratio of modified gypsum did not show any significant improvement in the strength values.

A possible explanation for this phenomenon is that the modified test groups with reduced L/P ratio still retained excess water in the gypsum mix, thus causing a decrease in the compressive and tensile strength. [15] It has been hypothesized that excess water retained in the set gypsum increases the volume of gypsum but reduces the strength values. [18]

Hence, further reduction in L/P ratio is recommended to improve the strength of the modified set gypsum.

Conclusions

The use of disinfectant solutions as water substitute for mixing gypsum adversely affected the compressive and tensile strength.

Amongst all the gypsum mixing solutions, water showed higher strength values, which was followed by sodium hypochlorite and glutaraldehyde in descending order of strength values.

The addition of 1% gum Arabic and 0.132% calcium hydroxide into the gypsum, lowered the L/P ratio.

The reduction in L/P ratio of modified gypsum did not significantly improve the strength properties when compared to that of the control groups.

There may be a future scope for improving the strength properties of modified gypsum by further reducing the L/P ratio.

Acknowledgment

The author would like to thank Dr. Kalavathy N and Dr. Sridevi J for their guidance and support in completion of this study. The author would also like to thank Dr. Soudhamini V Rao who has given positive inputs in preparing the article.

Infection control recommendations for the dental office and the dental laboratory. ADA Council on Scientific Affairs and ADA Council on Dental Practice. J Am Dent Assoc 1996;127:672-80. [PUBMED] [FULLTEXT]